Center of Mass: Tools and Techniques for Animating Natural Human Movement
May 20, 2010 Page 1 of 5
[In this Game Developer magazine reprint, a gymnast and UFC mixed martial art game developer dives into how the center of mass of a character is the basis for all realistic motion -- and delivers the technical know-how to achieve it.]
A common complaint from consumers regarding computer animation (especially in the video game industry) is the "puppet-like" movements characters often seem to perform. When two character models come into contact with each other in a fighting game, or when they must climb structures in an acrobatic manner, the difficulties become more pronounced.
In this article, I'd like to talk about some techniques for analyzing human movement that can be applied to make your hand-keyed character animations look more natural.
Takes One to Know One
Before joining the 3D computer software industry I was a competitive gymnast, and also worked as a gymnastics coach. When I was a gymnast, I would practice a move over and over to achieve the right timing, speed, balance, and angle.
I repeated and adjusted my technique until I could execute the move perfectly every time. I developed my skill to perform, analyze, and teach gymnastic moves through those experiences.
While I was coaching, I also studied physiology and kinesiology for sports science in university, focusing on the mechanism of the human body in movement. I learned how to analyze the physics of human motion using mathematical calculations, which lead to an understanding of how muscle generates the force to perform a move.
I was hired by Kung Fu Factory to look over the animation team to help them improve the quality of the company's character animation. Over the years I've come to realize that animating characters in 3D space is similar to coaching gymnastics, and my analysis of movement was quite relevant to the creation of quality animation. If you can analyze a move and understand the kinetics behind it, you can create better, more natural animation -- and faster, too.
The What and Why of Center of Movement
Some skilled animators have the ability to picture and recreate human movement easily. But most of us can't. At our studio we often encourage animators to use reference videos showing a person performing the action they're meant to animate. This helps them understand the movement better, but video can only go so far.
One critical, and often undervalued element of movement is an understanding of the character's center of gravity, or center of mass. I will refer to both as the "COM."
The center of gravity is the point at which the entire weight of a body may be considered as concentrated, so that if supported at this point the body will remain balanced in any position.
Your balance and movements are always affected by gravity. As a gymnast I learned how to control and adjust my body's COM to perform various actions. Tumbling, balancing, and so forth, all require an adjustment of the COM. Some animators I know who are martial artists also understand this concept, so they can see how the COM flows and adjust the character's body appropriately. Adjustment of the COM is something we all do naturally in real life when performing actions like dancing, running, and so forth -- but it's not easy to create this in a fictional character.
Figuring out the position of the COM is not hard when a human is standing up straight. Usually the COM of a human body sits around the lower stomach, which is a little above half his height. On a moving character it becomes more difficult, because the location of the COM will constantly shift.
For example, as you bend forward from a standing position, your COM will start to adjust and move forward as well. Eventually your COM will be so far forward that you'll fall over if you don't do something to regain your balance. This shifting of the COM position during the move represents the main flow of the movement you're performing (in this case bending over). We call it the COM trajectory.
If you can properly animate your characters with the appropriate COM trajectory, they will move much more realistically.
To do this, you have to know the physics behind the movement being performed, and how physics relates to the center of mass. An obvious example is gravity. Every object on Earth is affected by gravity. To understand how gravity affects the COM, physics calculations can be very useful. When I work with animators, I use Excel to show the physics simulation and how the movement is affected by gravity.
In the following section I will show you my process of physics simulation using Excel. I will also show a tool that displays the COM trajectory, used to examine if the COM is moving naturally.
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